Much attention and engineering has been performed to address the pressure drop that occurs as a result of fluid flowing into a wellbore. Solutions to minimize the pressure drop include such efforts as wellbore damage remediation, fracture stimulation, gravel packing and horizontal completions. All of these efforts attempt to address the pressure drop that occurs between the reservoir and the center of the wellbore.
In conventional, vertical wells the pressure drop occurring along the length of the completion is assumed negligible because the typical length of the completion is usually on the order of 10's of feet. This compares to the 1000's of feet of tubing between the wellhead and producing interval. However, for horizontal wells, the length of the completion can be as long as the vertical depth of the well. It is common industry practice to have horizontal completions that are 100's to 1000's of feet in length. Due to this substantially longer completion interval, for a horizontal well in comparison to the vertical well, the pressure drop occurring along the length of the completion is no longer insignificant.
The pressure drop along the length of the completion is sufficiently large to result in a non-uniform inflow of fluids along the length of the completion. An SPE (Society of Petroleum Engineering) Paper published in 1996 by Tang, Ozkan, Kelkar, Sarica and Yildiz shows the significance of this pressure drop. These findings are based on their work, which was organized as a joint industry project, titled: “Optimization of Horizontal-Well Completion II.” As presented in the SPE paper, the fluid flow into the wellbore is non-uniform. The highest contribution of fluid is at the heel of the completion. The fluid rate at the heel is more than four times the fluid rate at the center of the completion and almost two times the fluid rate at the toe of the completion. This variation in fluid inflow is due to the pressure drop resulting from non-parallel flow lines within the wellbore. The fluid flow paths literally collide with each other within the wellbore, which results in the turbulent-like fluid flow behavior.
New completion techniques designed to minimize the pressure drop that occurs due to the confluence of flow into the wellbore would be very desirable. Such techniques would be expected to provide the greatest benefit for horizontal wells due to the length of their completions. However, the techniques would also be beneficial for vertical wells with long and/or commingled completions.
By pursuing this objective, the new completion techniques should prove effective in 1) increasing the well's total productivity and 2) increasing the uniformity of inflow (conformance) along the length of the completion.